Heat storage tank with improved thermal stratification

ABSTRACT

Heat storage tank comprising an envelope ( 2 ) with a longitudinal axis (X) filled with a heat transfer liquid and solid heat storage elements, a first longitudinal end provided with first means ( 10 ) for collecting and supplying a liquid at a first temperature and a second longitudinal end provided with second means ( 12 ) for collecting and supplying a liquid at a second temperature, in which said solid heat storage elements are distributed across three beds (TH1, TH2 and TH3) superposed along the longitudinal axis (X), separated by a layer of liquid (L1, L2 and L3), the heat transfer liquid being capable of flowing from the first longitudinal end to the second longitudinal end.

TECHNICAL FIELD AND PRIOR ART

The present invention relates to a heat storage tank with improvedthermal stratification.

Numerous fields and numerous industrial applications implement thestorage of heat. The storage of heat enables the valorisation of heatstemming from industrial processes, the recovery of surplus energy ordissociating the moment of production of thermal energy from the usethereof.

As an example, in the CSP field (CSP designating “Concentrated SolarPower”), the surplus heat produced at times of strong sunshine may thusbe stored so as to be exploited at the end of the day.

The storage of heat may typically be realised either in the form ofsensitive energy (by varying the temperature level of a solid or liquidstorage material), in the form of latent energy (by changing the phaseof a storage material) or finally in the form of chemical energy (usingendothermic and exothermic chemical reactions).

In the case of sensible heat storage, the heat is stored by raising thetemperature of a storage material which may be liquid, solid or acombination thereof.

Industrial processes involving a use or a conversion of thermal energyby means of a thermodynamic cycle, for example by the use of a steamturbine, involve overall two temperature levels which are the conditionsat the limits of the cycle. It is sought to maintain these twotemperature levels as constant as possible in order to obtain optimisedoperation of the cycle. In fact, as an example, steam turbines, whichassure the conversion of thermal energy into electrical energy, havehigher efficiency when the input temperature in the turbine ismaintained constant at a predefined value. Consequently, storageassociated with such systems must thus respect these characteristics andmake it possible for example to destore heat at a constant temperaturelevel.

An example of this type of operation is the field of concentrated solarpower where a typical storage system consists of two tanks filled withstorage fluid at two temperature levels. One of the tanks stores at aconstant low temperature and the second storage tank at a constant hightemperature. The output temperature of the hot tank is thus constantthroughout destorage.

Systems only comprising a single tank containing both the hot fluid andthe cold fluid also exist. There then exists thermal stratificationwithin the tank, the hot fluid situated in the upper part and the coldfluid situated in the lower part are then separated by a transitionregion known as “thermocline”.

The use of a single tank makes it possible to reduce the number ofcomponents, such as pumps, valves, etc. and to simplify command-control.

In thermocline type storage, the storage material may be a heat transferliquid or, advantageously, a mixture of a heat transfer fluid and acheap solid material. The use of such a solid material furthermore makesit possible to improve the segregation of the hot fluid and the coldfluid while reducing remixing effects. In the latter case, this is thenreferred to as “dual thermocline” (or “mixed-media thermocline”).

This “dual thermocline” tank has the advantage of reducing the quantityof liquid necessary, given that solid rock type materials are cheap, thetotal cost is reduced.

In a thermocline tank, in order to take account of density differencesand to avoid natural convection movements, the heat transfer fluid isintroduced via the top of the tank during storage phases and via thebottom of the tank during destorage phases. Storage is thuscharacterised by a hot zone at the top of the vessel, a cold zone at thebottom and a transition zone between the two zones known as athermocline. The principle of this type of heat storage is to create a“heat piston”, that is to say the advance of a thermal front that is asthin as possible and uniform transversally. This makes it possible tomaintain constant temperatures during charge and discharge phases.

During charge phases, cold liquid is removed from the tank via thebottom and is heated, for example by passing through a heat exchanger ofa solar collector, and then sent back into the tank via the top. Duringdischarge phases, hot liquid is removed from the tank via the top, andis sent for example to the evaporator of a thermodynamic cycleincorporating a turbine, in which it is cooled and is then sent backinto the tank via the bottom. During charge and discharge phases theheat piston moves downwards and upwards respectively.

“Dual thermocline” type storage based on a mixture of liquid heattransfer fluid and solid matrix brings into play very low fluidvelocities of the order of several mm/s in order to assure the transferof heat between the fluid and the static charge and to limitinhomogeneities.

Thermocline tanks using a mixture of a liquid heat transfer fluid and asolid matrix are subject to the problem of “thermal ratcheting”: duringheating phases, the vessel expands and the solid matrix descends tooccupy the freed space. During cooling phases, the vessel contracts andis constrained by the packed bed of rocks. The dimensioning of a vesselfor dual thermocline storage must therefore find an equilibrium between:

-   -   mechanical strength, linked to the thermal ratcheting which        guides towards rather flat geometries, i.e. large tank diameter        and small tank height in order to reduce the height to diameter        ratio;    -   hydraulics, which guide towards cigar type geometries, i.e.        small tank diameter and large height, in order to favour        homogeneous distribution of heat transfer fluid and to retain a        thin and transversally uniform heat piston.

In real operation, such a storage system has inhomogeneities and theheat piston is not perfect. These inhomogeneities can stem from:

-   -   inhomogeneities in the distribution of the static charge which        may be linked to the initial filling of the storage tank        (non-homogeneous mixture, segregation of the rock and sand etc.)        or to the thermal cycling of the solid matrix which “comes        alive” during thermal expansions and contractions of the tank;    -   edge effects at the level of the tank wall. These edge effects        are of the hydraulic type, since the wall induces inhomogeneity        of the static charge, and of the thermal type due to a cold wall        in contact with hot fluid in charge phase and a hot wall in        contact with cold fluid in discharge phase;    -   poor distribution of the heat transfer fluid in the static        charge.

These inhomogeneities lead to the appearance of preferential paths, withchimney effects which degrade the heat piston operation and restrict thecorrect operation of the thermocline. In charge phase, it may happenthat there are hot “tongues” progressing in the cold fluid. A hightemperature disparity then appears in a transversal plane of the tank.This leads, for example, to the output temperature of the tank during adischarge phase being constant over a smaller time range, which isundesirable for the thermodynamic conversion unit.

DESCRIPTION OF THE INVENTION

It is consequently an aim of the present invention to offer a “dualthermocline” type of heat storage tank having homogeneous transversaltemperature distribution, so as to arrive at very stable hot temperatureand cold temperature values.

The aim of the present invention is attained by a tank comprising asolid matrix and a heat transfer liquid distributed in several stages influid communication, each stage comprising a layer of solid matrix, thelayers of solid matrix of two consecutive stages being separated by alayer of heat transfer liquid in which natural convection movementsarise in the case of temperature inhomogeneity in a transversal plane.These natural convection movements assure homogenisation of thetemperature, which makes it possible to re-establish transversaltemperature homogeneity in the beds of solid matrix.

As an example, in charge phase, in each stage, while progressing in thelayer of solid matrix from the top to the bottom of the tank transversaltemperature inhomogeneities arise in the thermal front. When the thermalfront encounters the layer of heat transfer liquid of the lower stage,due to natural convection movements, these transversal inhomogeneitiesare lessened. Thus, at each passage from one step to the other, thetransversal temperature inhomogeneities are lessened, which makes itpossible to maintain a heat piston.

In other words, the storage tank is compartmentalised over its height bymeans of elements capable of allowing the liquid to circulate, in orderto create under each element purely liquid zones above solid zones ofheat storage material. By virtue of the very low fluid velocities and asolid static charge, the liquid zones thereby created make it possibleto reduce inhomogeneities by natural convection mechanisms and thus to“re-initialise” the heat piston at each passage from one compartment tothe next.

The elements delimiting the compartments are for example grates.

Preferentially, the solid zones comprise elements with at least twoparticle sizes, making it possible to reduce the empty spaces of thesolid matrix and thus the quantity of heat transfer liquid necessary.

The phenomenon of “thermal ratcheting” is advantageously reduced, sinceeach compartment has a low height with respect to its diameter whileassuring a transversally uniform heat piston since the tank has a largeheight compared to its diameter.

The subject-matter of the present invention therefore is a heat storagetank comprising an envelope with a longitudinal axis filled with a heattransfer liquid and solid heat storage elements, a first longitudinalend provided with first means for collecting and supplying a liquid at afirst temperature and a second longitudinal end provided with secondsmeans for collecting and supplying a liquid at a second temperature, inwhich said solid heat storage elements are distributed across at leasttwo beds superposed along the longitudinal axis, separated by a layer ofheat transfer liquid, the heat transfer liquid being capable of flowingbetween the first longitudinal end and the second longitudinal end. Forexample, each bed rests on a support enabling fluid communication.

At least one of the supports may comprise a bearing structure and aslatted structure covered with a metal web plate.

The supports are preferably in two parts.

The solid heat storage elements have advantageously at least twodifferent particle sizes.

The layer of heat transfer liquid preferably has a thickness comprisedbetween 1 cm and 10 cm.

For example, the envelope is a shell and the height of each bed is lessthan the diameter of the envelope.

The solid heat storage elements may comprise blocks of rocks and sand.The blocks of rock are formed for example from alluvial rocks. The heattransfer liquid is for example a thermal oil.

The first and/or the second collecting and supplying meansadvantageously comprise distribution means assuring transversalhomogeneity of the axial velocity of the fluid.

The envelope may be a shell. The second distribution means may comprisea supply duct extending along the diameter of the shell and distributionducts extending laterally from the supply duct, said distribution ductsbeing provided with orifices distributed along the length thereof.Advantageously, the distribution ducts have different lengths such thatthe contour of the distribution means has substantially the shape of acircle.

The second supplying and collecting means may be isolated from the solidheat storage elements.

Another subject-matter of the present invention is a solar power plantcomprising at least one heat tank according to the invention.

The solar power plant may be a Fresnel type solar power plant or a towersolar power plant.

The first and the second means for collecting and supplying the tank maythen be connected to a turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by means of thedescription given hereafter and the appended drawings in which:

FIG. 1 is a longitudinal sectional view of an example of embodiment of aheat storage tank according to the invention,

FIG. 2 is a detailed view of the tank of FIG. 1 representedschematically illustrating the operation of the tank,

FIGS. 3A and 3B are top views of an example of embodiment of supportsintended to delimit the compartments in the tank,

FIG. 3C is a sectional view along the plane A-A of FIG. 3A,

FIG. 4 is a top view of an example of embodiment of a distributor whichmay be implemented in the tank according to the invention,

FIG. 5A is a schematic longitudinal sectional view of a tank accordingto the invention in which are represented temperature measurementplanes,

FIGS. 5B and 5C are transversal sectional views of the distribution ofthermocouples in the heat transfer liquid and in the different planes ofthe solid bed respectively,

FIGS. 6A to 6D are graphic representations of temperature measurementsprovided by the thermocouples in the different levels of the differentstages of the tank of FIG. 5A.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

In the description hereafter, the terms “stage” or “compartment” will beused indiscriminately.

The terms “lower”, “upper”, “top” and “bottom” are considered withrespect to the orientation of the tank in FIG. 1.

In FIG. 1 may be seen a longitudinal sectional view of an example ofheat storage tank according to the invention.

The tank comprises a cylindrical envelope 2 with a longitudinal axis X.In the example represented, the tank has a circular section. Thelongitudinal axis X is intended to be oriented substantially verticallyas in the representation of FIG. 1.

The envelope 2 is formed of a shell 4 and two convex bottoms 6, 8closing the upper and lower longitudinal bottoms respectively of theshell 4.

The tank comprises means for admitting and collecting 10 hot liquidsituated in the upper convex bottom 6 of the tank and means foradmitting and collecting 12 cold liquid situated in the lower convexbottom 8 in the lower part of the tank.

The inside of the tank is divided into several compartments C1, C2, C3superposed along the longitudinal axis X. Each compartment C1, C2, C3comprises an bottom G1, G2, G3 forming support assuring the retention ofthe solid heat storage elements while enabling fluid communicationbetween the compartments and a bed TH1, TH2, TH3 of solid heat storageelements. Only the bed TH1 is represented by solid elements.

Moreover, a layer of heat transfer liquid L1, L2, L3 covers the bedsTH1, TH2, TH3 of solid heat storage elements.

The active volume of the tank does not comprise an empty zone, such thatthe volume not occupied by the solid elements is filled with the heattransfer fluid. The zone situated above the bed TH1 and delimited by theconvex bottom 6 is not filled with liquid and forms a crown for theevacuation of vapours.

In the example represented, the zone situated under the bed TH3delimited by the lower convex bottom 8 is filled with liquid of a solidmaterial, for example of concrete type. In addition, this makes itpossible to reduce the quantity of heat transfer fluid implemented.

The heat storage elements are formed for example of rocks and/or sand.Preferably, the elements have at least two particle sizes therebyassuring good filling and reducing the free spaces for the heat transferliquid. Advantageously, the solid heat storage elements are formed ofblocks of rocks and sand filling the spaces between the rocks.

Each particle size corresponds to a diameter d50 of solid elements,defined as the value for which 50% of the solid elements have a diameterless than d50. The diameter d50 is also designated the median.

Preferably, a factor 10 between the medians of the two particle sizes ischosen which enables the filling of the free space between large rocksby small rocks. For example, the large rocks have a diameter of around 3cm and the small rocks have a diameter of around 3 mm. The distributionby volume is as follows: around 75% of large rocks and 25% of smallrocks by volume.

They may be, for example, alluvial rocks mainly composed of silica. Therocks are chosen as a function of their characteristics linked to theheat storage capacity and to their thermal behaviour (density, specificheat capacity and thermal conductivity) and to their compatibility withthe heat transfer liquid, for example the compatibility between thegeological nature of the rock and the heat transfer liquid.

The heat transfer liquid is for example oil or molten salts. Forexample, the oil may be Therminol66® or Jarytherm DBT®, this not showingany particular interactions with alluvial rocks, more generally hightemperature synthetic thermal oils may be suitable in use with alluvialrocks.

For the sake of simplicity, the “beds of solid heat storage elements”will be designated hereafter “heat storage beds”.

The supports are thus adapted to support mechanically the heat storagebeds, to retain the elements of low particle size, such as sand, and toallow the heat transfer liquid to pass through.

In FIGS. 3A to 3C may be seen details of a support F1 according to anembodiment example.

Advantageously, the support G1 is formed of two half-supportsfacilitating its mounting in the shell 4. A support in one piece doesnot go beyond the scope of the present invention.

In FIGS. 3A and 3B may be seen a bearing structure 14 in the shape of ahalf-disc, and a slatted structure 16 in the shape of a half-disccovered with a grate 18 resting on the bearing structure 14.

The bearing structure 14 is formed of parallel bearing bars 20 securedto one another by cross-pieces 22 and forming a structure in the shapeof a half-circle.

In FIG. 3C may be seen a sectional view along the plane A-A of FIG. 3Aof the slatted structure 16 and the grate 18.

The grate 18 is for example formed of a metal screen in which the meshsize is such that it assures the retention of the solid elements of thesmallest particle sizes.

Each support G1, G2, G3 is suspended in the shell by means of an annularlug 23 lining the inner surface of the shell at the desired height.

The admitting and collecting means 10, 12 preferably comprise an orificeto collect the hot and cold fluid respectively and distribution means tosupply the tank with hot and cold fluid respectively.

In FIG. 4 may be seen an advantageous example of embodiment ofdistribution means seen from the top.

The distribution means 24 comprise a supply duct 26 connected to theexternal liquid supply and distribution ducts 28 connected to the supplyduct and extending transversally with respect thereto. In the examplerepresented, the distribution ducts 28 are perpendicular to the supplyduct 26. Each duct is provided with a plurality of distribution orificesassuring a distribution of the liquid along its axis.

The main duct extends advantageously along a diameter of the shell. Alsoadvantageously, the distribution ducts have different lengths as afunction of their position along the main duct such that thedistribution means cover in a substantially homogeneous manner theentire transversal section of the shell.

Other forms of distribution means may be envisaged, preferably theseforms assuring a homogeneous distribution of the liquids supplying thetank.

In FIG. 2 may be seen represented schematically an upper part of acompartment C2 and a lower part of the compartment C1.

The layer of liquid L2 may be seen above the heat storage bed TH2 andbelow the support G1 which is covered with the heat storage bed TH1.

The arrow F symbolises the natural convection movements that arise inthe layer of liquid L2 when it is the site of transversal temperatureinhomogeneities.

In the case of transversal temperature inhomogeneities, temperaturegradients and thus liquid density gradients arise in the liquid layers,which leads to the appearance of natural convection movements which tendto reduce this gradient.

Preferably, the thickness of the liquid layers is of the order of 1 cmto 10 cm.

It has been observed that for thicknesses less than 1 cm, the overallremixing function is less well assured because the convection cells thatare created have a more local effect.

For thicknesses greater than 10 cm, the efficiency of the remixingfunction is maintained. Conversely, the higher the thickness of thelayer of heat transfer liquid the greater the quantity of liquid. Yetthe cost of the liquid is high. As a result, a tank with liquid layershaving a considerable thickness is economically less interesting.Furthermore, too high thicknesses of liquid layers would result in a tooimportant axial remixing which would reduce the efficiency of thethermocline.

Advantageously, the compartments all have substantially the same heightand the same composition in quantity of liquid and in quantity of solidelements so as to assure homogeneous behaviour over the whole height ofthe tank.

The height of the storage tank is thus “cut up” into several regions ofheight hi: h1, h2, h3 which can vary from several tens of centimetres toseveral metres. The height of the beds is in practice chosen so as toconserve a ratio hi/D<1, which makes it possible to reduce themechanical phenomenon of thermal ratcheting.

As an example uniquely, a tank having a low temperature of 150° C. and ahigh temperature of 300° C., may comprise a shell having a diameter of2500 mm, three compartments each comprising a heat storage bed of heightequal to 1900 mm and a layer of heat transfer liquid having a thicknessof 100 mm.

The efficiency of the structure of the tank according to the inventionwill now be shown.

For this, a tank with four compartments is considered. Temperaturemeasurements are carried out in the liquid layers L1 to L3 and atdifferent heights in each heat storage bed TH1, TH2, TH3, TH4 and atseveral points of transversal planes of each bed corresponding to thedifferent heights. The different measurement heights are represented inFIG. 5A. The different measurement points per height are represented inthe section of FIG. 5C for the rocks and in the section of FIG. 5B forthe liquid. The measurements are performed by means of thermocouples.

The measurements are represented in the graphs of FIGS. 6A to 6D foreach of the compartments C1 to C4 respectively.

The charge is carried out at the temperature of 170° C. and the tank isinitially entirely at the temperature of 60° C.

The advance of the thermal front is symbolised by the arrow Fth in thegraphic representations.

Analysis of the temperature measurements shows that in the uppercompartment C1, three groups of curves may be distinguishedcorresponding to the plane C1-3, to the plane C1-2 and to the planeC1-1. As the thermal front progresses in the shell along the axis X, atemperature inhomogeneity appears: in fact it is observed that thecurves are less and less grouped together, which reflects the existenceof temperature differences between measurement points situated on a sameplane. The inhomogeneity of the temperature thus increases from theplane C1-1 to the plane C1-3.

The passage from the compartment C1 to the compartment 2 results in astricture of the curves in the plane C2-1 compared to those of the planeC1-3 (FIG. 6A), which signifies a reduction in the inhomogeneity of thetemperature in the plane C2-1 compared to the plane C1-3.

The passage by the liquid layer L2 between the beds TH1 and TH2 makes itpossible to reduce the spreading out of the group of curves, that is tosay to reduce the temperature inhomogeneity.

The same phenomenon appears at each of the passages from one compartmentto the other during the passing through of a liquid layer.

In the compartments C3 and C4, only two groups of curves are observed: afirst group well compressed together corresponding to the planes C3-1and C4-1 and a group of spread out curves corresponding to the followingtwo measurement sheets C3-2 and C3-3 and C4-2 and C4-3. This illustratesan inhomogeneity in the tank due to the bed of rocks. Nevertheless, thepassage by the layer liquid L4 makes it possible to re-establishtemperature homogeneity.

In a tank according to the invention, the reduction of the temperatureinhomogeneities during the passage by a uniquely liquid layer has beenobserved experimentally even in the case of considerable temperatureinhomogeneity in a plane situated upstream of the liquid layer. Theliquid layer also makes it possible to delay the destabilisation of thethermocline since the temperature dispersion on the planes C2-1, C3-1and C4-1 is lower than on the plane C1-3.

The tank then has an improved operation which comes close to heat pistonoperation. The tank according to the invention thus helps in maintaininga constant temperature at the outlet of the tank.

Furthermore, the active proportion of the tank is increased. This isbecause the reduction of transversal temperature inhomogeneities makesit possible to obtain a greater volume percentage of the tank atconstant temperature.

Moreover, thanks to the invention, it is possible to combine theadvantages of a low bed height to shell diameter ratio and a high totalheight to shell diameter ratio.

This is because the segmentation of the bed of solid elements makes itpossible to attain, for each compartment, a heat storage bed height toshell diameter ratio less than 1, which makes it possible to reduce theeffect of thermal ratcheting and thus assure good mechanical strength.And, simultaneously, the segmentation makes it possible to have aconsiderable total height of solid element bed and thus a high totalheight to diameter ratio. Important storage properties in terms ofduration and volume of isothermal zone are thereby obtained.

Furthermore, thanks to the invention, it is possible to reduce thethickness of the shell compared to those of the prior art since thethrust linked to the solid storage materials is distributed in thedifferent compartments. Moreover, the phenomenon of packing down duringthermal cycles is spread out in the different compartments.

Moreover, thanks to the distribution in compartments, the distributionmeans situated in the lower bottom of the tank are isolated from thesolid heat storage elements, they are then no longer subjected tomechanical stresses linked for example to the packing down of thismatrix during thermal cycles.

The tank according to the invention may be used for storing the heat ofany installation or system producing heat.

It is particularly suited to use with systems using liquids havingcontrolled and constant temperatures, such as turbines.

The tank according to the present invention is particularly suited touse in a Fresnel type solar power plant to supply a turbine. It may alsobe used in a tower solar power plant.

What is claimed is: 1-18. (canceled)
 19. Heat storage tank comprising anenvelope with a longitudinal axis filled with a heat transfer liquid andsolid heat storage elements, a first longitudinal end provided with afirst means for collecting and supplying a liquid at a first temperatureand a second longitudinal end provided with seconds means for collectingand supplying a liquid at a second temperature, in which said solid heatstorage elements are distributed across at least two superposed alongthe longitudinal axis, separated by a layer of heat transfer liquid, theheat transfer liquid being capable of flowing between the firstlongitudinal end and the second longitudinal end.
 20. Tank according toclaim 19, in which each bed rests on a support enabling fluidcommunication.
 21. Tank according to claim 20, in which at least one ofthe supports comprises a bearing structure and a slatted structurecovered with a metal web plate.
 22. Tank according to claim 20, in whichthe supports are in two parts.
 23. Tank according to claim 19, in whichthe solid heat storage elements have at least two different particlesizes.
 24. Tank according to claim 19, in which the layer of heattransfer liquid has a thickness comprised between 1 cm and 10 cm. 25.Tank according to claim 19, in which the envelope is a shell and inwhich the height of each bed is less than the diameter of the envelope.26. Tank according to claim 19, in which the solid heat storage elementscomprise blocks of rocks and sand.
 27. Tank according to claim 26, inwhich the blocks of rock are formed from alluvial rocks.
 28. Tankaccording to claim 19, in which the heat transfer liquid is an oil. 29.Tank according to claim 19, in which the first and/or the secondcollecting and supplying means comprise a distributor assuringtransversal homogeneity of the axial velocity of the fluid.
 30. Tankaccording to claim 29, in which the envelope is a shell and the seconddistributor comprises a supply duct extending along the diameter of theenvelope and distribution ducts extending laterally from the supplyduct, said distribution ducts being provided with orifices distributedalong the length thereof.
 31. Tank according to claim 30, in which thedistribution ducts have different lengths such that the contour of thedistributor has substantially the shape of a circle.
 32. Tank accordingto claim 1, in which the second supplying and collecting means areisolated from the solid heat storage elements.
 33. Solar power plantcomprising at least one heat tank according to claim
 19. 34. Solar powerplant according to claim 33, in which the solar power plant is a Fresneltype solar power plant.
 35. Solar power plant according to claim 33, inwhich the solar power plant is a tower solar power plant.
 36. Solarpower plant according to claim 33, in which the first and the secondmeans for collecting and supplying the tank are connected to a turbine.